Efficiency improvements in compounding extruders

Areas for improving efficiency in compounding processes include improving energy efficiency, increasing output of quality product by optimizing screw design, and minimizing raw material waste that occurs during start-up or changeover.

In order to improve energy efficiency, processors should first be sure to use an extruder of the proper size and screw design for the required output rate and product formulation. Using large extruders for low output wastes energy, because motors run most efficiently close to their designed maximum speed, notes Dr. Robin Kent, managing director of Tangram Technology, an energy management consultancy specializing in plastics processing. Frequent preventive maintenance can lead to energy savings. Specific energy (measured in kWh/kg) is a sensitive indicator of screw and barrel wear, says Dr. Kent. He explains: “If you see the specific energy of the same product on the same line going up over time, you should check to see what maintenance is needed.”
The new Coperion Melt Pump (MP) is being used in large scale polyolefin compounding and pelletizing operations, where it offers greater energy efficiency for melt pumping than using an extruder alone and may be used to debottleneck the compounding process. Operations that need to pump melt through downstream equipment and large underwater pelletizers can use the Coperion MP to reduce energy usage by 20%, says Paul Andersen, director of process technology at Coperion. The magnitude of the reduction depends on the polymer and initial pressure drop through the downstream equipment, he explains.
Coperion’s new ZSK-NT two-stage, twin screw extrusion system for compounding challenging products such as bimodal PE has about 10% higher energy efficiency as well as significantly improved quality and about 10% higher capacity for this application than the conventional ZSK and melt pump, reports the company. The system separates the melting step from the homogenization step using two ZSK MEGAcompounders, with the first running at high screw speeds for rapid melting and the second running at slow speeds for homogenization and pressurization. Screw configurations and process conditions are optimized for the two stages, which reduces the total energy input and improves melting of the high-molecular weight portion of the polymer without overheating or degrading the low-molecular weight portion.
Variable speed drives can offer improved energy efficiency to compounding operations. Coperion offers a planetary gear design for very large machines (10,000-20,000 hp or 7500-15000 kW). The gearbox has two drives – a large (10,000-15,000 hp or 7500-11,000 kW) single speed drive and a variable speed, planetary gear (3000-5000 hp or 2200-3700 kW) that allows incremental variation of the top screw speed. This flexibility increases energy efficiency by allowing screw speed to be optimized for the material being processed, explains Dr. Andersen.
Direct drive systems that are directly coupled to the screw without a gear reducer offer potential energy savings for single screw extruders, allowing higher speed extrusion for a given energy input. American Maplan’s CMG (Compact Motor Gear) four-motor direct drive is being used in single-screw direct extrusion of PS and PET sheet, as well as in single screw compounding. Energy optimized direct drives and high speed screw designs, offered by several extruder manufacturers, are currently being used mostly in extrusion processing. For example, ETA Kunststofftechnologie GmbH’s energy optimized, high speed, grooved barrel extruder reduces energy loss using a direct drive and a barrier mixing screw that has a lower cooling demand in the grooved feeding zone compared to traditional grooved feed designs. The screw is also designed to control melt temperature at high screw speeds using deep channels that lower frictional heating, explains Dr. Robert Michels, director of technology and projects at ETA. Although ETA screws are used primarily for processing, they are also used in a few direct extrusion applications such as mixing calcium carbonate masterbatch into PP for sheet extrusion.

Optimizing screw design and maximizing output

Twin screw extruders
For twin screw compounding extruders, an important goal is to optimize screw design in order to transfer energy to mixing most efficiently. Different screw element designs can be used depending on the material processed. For example, when introducing a liquid into the melt, a toothed element is more efficient at mixing very low and very high viscosities than a kneading block, points out Dr. Andersen.
High speed twin screw extruders (TSE) allow higher throughput on smaller extruders through a combination of shaft design and improved materials of construction that allow higher torque and higher speeds. For example, NFM’s new, high-speed TSE can run up to 1500 rpm, compared to 600-900 rpm for older technology, says Butch Noll, national sales manager for extruder manufacturer NFM Welding Engineers, Inc. Because this design generates high shear rates, it is not intended for heat or shear sensitive resins, but is useful for highly filled compounds or colour concentrates, for example. NFM’s TEM Series is being used to produce pelletized concentrates with high fill levels, such as 80-90% calcium carbonate. While these high fill levels had generally been produced in two-part processes such as a continuous mixer and a single screw extruder, high speed TSE can produce these formulations at similar outputs but greater energy efficiency than using two machines. One of NFM’s customers, who was able to replace larger extruders with smaller, high-speed extruders, even chose to reduce the overall size of their building, allowing them to save in utility costs, notes Mr. Noll.
The Leistritz MAXX high speed twin screw design, introduced in 2004, has a larger outside diameter/inside diameter ratio (Do/Di) of 1.66/1 compared to traditional screws with a Do/Di of about 1.55/1, which increases the free volume by about 30%. The MAXX design uses a patent-pending asymmetrical splined shaft to maintain the same shaft torque despite the smaller diameter shaft, allowing higher throughputs. An asymmetrical geometry transmits power more efficiently into the screw elements by eliminating radial force and increasing tangential force, explains Charlie Martin, president of American Leistritz. To handle higher volumes and output rates, barrel cooling efficiency was increased by adding cooling bores and increasing coolant flow rate. The increased volume is particularly beneficial for feeding low bulk density materials, notes the company.
Coperion’s ZSK MEGAvolume high speed compounding extruder is targeted for low bulk density materials that do not require high energy input but do require high volume. The extruder has a smaller diameter shaft to accommodate deeper screw flights, with a Do/Di ratio of 1.8, allowing 40% greater free volume. The MEGAvolume extruder runs at about 40% lower average shear rates than Coperion’s high torque equipment, which is beneficial for heat and shear sensitive compounds. The lower average shear rate also permits operation up to 1800 screw rpm. While the MEGAvolume has lower torque than the MEGAcompounder, future developments could result in increased power capabilities, predicts Dr. Andersen.

Single screw compounding extruders
Screw design is important in single screw extrusion for minimizing energy consumption and improving mixing quality. Power used to turn the screw is dissipated into the material through heat, and excess heat must be removed through barrel cooling. Ideally, the extruder should be designed to input just the right amount of energy to melt and mix. Rauwendaal Extrusion Engineering Inc. (REE)’s CRD mixers and barrier screw design for single screw compounding extruders improve mixing while consuming less energy. CRD mixers create elongational flow using tapered slots in the flights and slanted pushing flight flanks, explains Chris Rauwendaal, president of Rauwendaal Extrusion Engineering. Elongational flow is more efficient than shear flow, because elongational flow dissipates less energy into the material through heat. Elongational flow improves both distributive and dispersive mixing. CRD barrier screws add a slanted section to the traditionally flat crest of the barrier flight, creating a wedge-shaped region. Flat crests create shear flow, resulting in high power consumption, high viscous dissipation and high melt temperatures, which reduce viscosity and dispersive mixing. In contrast, the wedge-shaped region generates elongational flow, which allows more effective mixing because it reduces power consumption and viscous dissipation, yet increases the stresses in the melt, explains Dr. Rauwendaal.

Other continuous compounding equipment
The Buss kneader, for compounding PVC and other shear- or temperature-sensitive polymers, offers lower specific energy use than typical twin screw extruders, partly due to its mixing technology that has controlled shear rather than high peak shear. The new Buss 4-flight, quantec® Kneader has two to three times the output of previous 3-flight Kneader designs for the same screw diameter. Throughput is increased due to a 4-flight screw design that improves material transport, a larger intake zone for feeding low-bulk density powders, a new screw geometry, and a more efficient use of the drive that can run up to 750 rpm. At the end of 2006, Buss introduced an expanded volume (EV) version of the quantec with 20% more free volume, resulting in even better dispersion and additional throughput capability. The expanded volume has been so successful that Buss made it standard for new quantec machines. Buss also offers EV retrofit kits, says Mike Irish, vice-president of Buss Kneader Technology at Buss, Inc., USA. The quantec’s higher output and skid-based design provides the opportunity to save space by using a smaller footprint machine to obtain the necessary output. In addition to production efficiency, the 4-flight quantec design also has higher energy efficiency than the 3-flight design. “The new quantec has more shearing cycles and less backflow. Less cooling of the melt is needed at the end of the process, again resulting in energy savings” explains Colin Richardson, technical marketing manager at Buss. The quantec is being used in PVC compounding and pelletizing, PVC calendered film, thermoset powder coating production, and compounding of speciality XLPE and similar scorch-sensitive applications.
American Maplan’s planetary extruder, which offers low shear and highly dispersive mixing, is being used in direct extrusion of PVC calendered film as well as in PVC compounding. In some cases, processors are finding that an additional benefit of highly dispersive mixing is improved dispersion that allows them to reduce additive levels but maintain properties and stability, explains Paul Godwin, Composites and Compounding sales manager at American Maplan.
The Farrel Continuous Mixer (FCM) has long been recognized as good for energy efficient compounding, particularly of high filler levels. For example, in tests comparing the FCM to Farrel’s TSE, the FCM exhibits 30-35% better energy efficiency in a 40% talc filled PP and nearly 100% improvement in an 80% calcium carbonate filled PE, says Bill Flaherty, vice-president of sales and marketing at Farrel. In a series of developments over the last year, Farrel has obtained higher speeds, torque, and power in the FCM to maximize throughput and allow very high fill levels up to 80%, adds Mr. Flaherty.

Minimizing downtime

Small batches and frequent transitions are common in the compounding industry. “Since in most instances processors can’t reduce the number of transitions without increasing inventory, the goal should be to minimize the time it takes to transition,” comments Adam Dreiblatt, president of extrusion consultancy Extrusioneering International, Inc. Operator training is one key to minimizing downtime and scrap. “Training staff provides dramatic improvements in efficiency very quickly and for a relatively low investment,” says Mr. Dreiblatt. On the equipment side, manufacturers have introduced designs that allow quick cleaning to reduce changeover time. Developments in lower-cost PLC (programmable logic controllers) available for compounding processes allow advanced data acquisition, recipe management capability, network connectivity, and automated temperature and process control. Automated control systems allow quick and reproducible start-ups. Increasing the lifetime of extruder parts through improved metallurgy can add to uptime by reducing maintenance.
Extrusion simulation software can be used to minimize the start-up time needed to make quality product when running a new formulation or using new equipment. As raw material prices increase, use of virtual, simulated production trials to replace actual production trials is growing, says Mr. Dreiblatt. Simulation software can be used to optimize extrusion conditions in scaling-up from a small line to a larger one, in moving production from one plant to another, or in beginning production on new equipment. PolyTech’s latest TXS™ software, distributed by Extrusioneering, includes simulation of new, high speed and higher free-volume designs.

Contacts:
American Maplan; www.maplan.com
Buss Inc. - USA; www.busscorp.com
Coperion Werner & Pfleiderer GmbH; www.coperion.com
ETA Kunststofftechnologie GmbH; www.eta-gmbh.de
Extrusioneering International, Inc.; www.b4uextrude.com
Farrel Corp.; www.farrel.com
Leistritz; www.leistritz-extrusion.com
NFM Welding Engineers Inc.; www.nfm.net
Rauwendaal Extrusion Engineering; www.rauwendaal.com
Tangram Technology; www.tangram.co.uk